The choroid is the posteriormost portion of the uvea and is a highly
vascularized and pigmented tissue embedded between the retinal pigment
epithelium and the sclera.
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The thickness of the choroid is greatest at the posterior pole, where it
is about 0.2 mm thick. In the periphery, the choroid tapers to a thickness
of about 0.1 mm.
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The choroid adheres only loosely to the sclera over most of its area, and
as a consequence, the choroid is easily separated from the sclera by either
hemorrhage or serous fluid. The only points at which the choroid is fixed
are the optic nerve and at the vortex veins. Attachment of the choroid
to the sclera at the vortex veins explains the classic quadrantic appearance
of choroidal detachments.
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The choroid receives its blood supply primarily from the posterior ciliary
branches of the ophthalmic artery. There is also some supply from the recurrent
anterior ciliary arteries. Four or five vortex veins provide the choroid's
venous drainage. The nerve supply of the choroid is derived from the long
and short ciliary nerves. The long ciliary nerves are branches of the nasociliary
nerve, which originates from the first division of the trigeminal nerve.
The long ciliary nerves carry sensory and sympathetic fibers. The short
ciliary nerves arise from the ciliary ganglion of the oculomotor nerve
and carry both sympathetic and parasympathetic fibers.
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The choroid is a multilaminated structure that can be divided into four
layers: Haller's layer, Sattler's layer, choriocapillaris, and Bruch's
membrane. Haller's layer consists of the large-diameter vessels of the
choroid and represents the outermost layer of the choroid.
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Interspersed with the blood vessels are melanocytes, nevus cells, and nerves.
These blood vessels possess an outer adventitial layer, an intermediate
smooth muscle layer, and an internal elastic lamina. They are not fenestrated
and as a result do not leak dye during fluorescein angiography.
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Sattler's layer is deep to Haller's layer and is composed of medium-sized
blood vessels, melanocytes, fibroblasts, lymphocytes, mast cells, and supporting
collagen fibers. Within Sattler's layer, arteries gradually decrease in
caliber and form arterioles. In the process, the arteries lose their muscularis
layer and their internal elastic laminae. Like the vessels of Haller's
layer, the vessels of Sattler's layer are not fenestrated and do not leak
fluorescein.
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The choriocapillaris is a layer of capillaries that is immediately adjacent
to Bruch's membrane. The diameter of the capillaries is relatively large
and measures between 25 and 50 µm.4 These capillaries are fenestrated
and therefore are permeable to large molecules, including fluorescein.
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On fluorescein angiography, the diffuse fluorescence of the choroid is
primarily due to the leakage of fluorescein from the choriocapillaris.
The choriocapillaris extends from the edge of the optic disc to the ora
serrata.
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